1. Field of the Invention
The present invention relates to a process for desulfurizing molten pig iron, and more particularly to an improved process for desulfurizing molten pig iron.
2. The Prior Art
It is well known to use CaO as a desulfurizing agent in the desulfurization of molten pig iron, but CaO has been of limited use because of its poor desulfurization efficiency, as compared with other desulfurizing agents, such as CaC.sub.2.
According to the studies made by the present inventors, the poor desulfurization efficiency of CaO seems to be due to the following fact. Silicon, inevitably present in molten pig iron, is oxidized on the surfaces of the injected particles of CaO desulfurizing agent, forming shells of calcium silicate (2CaO.SiO.sub.2, 3CaO.SiO.sub.2) of high melting point on the surfaces of the particles and this interferes with the progress of the desulfurization reaction.
As a means of utilizing cheap CaO as a desulfurizing agent to reduce the desulfurization cost, the present inventors have found that injecting aluminum alone into the molten pig iron before injecting the CaO desulfurizing agent into the molten pig iron will improve the desulfurization efficiency of CaO. This concept is disclosed in Japanese patent application Ser. No. 102619/77. More specifically, Japanese patent application Ser. No. 102619/77 concerns a process for desulfurizing molten pig iron, characterized by first adding aluminum to molten pig iron, thereby adjusting an aluminum concentration to a specific one with respect to the silicon in the molten pig iron and to the sulfur to be removed. This addition of aluminum suppresses oxidation of silicon and supplements the aluminum which is consumed in the desulfurization reaction of the molten pig iron. Then, CaO having a given size below a specific size is added as a desulfurizing agent to the molten pig iron for desulfurization. The process of adding aluminum alone in advance of the CaO forms calcium aluminate which can prevent formation of the shells of calcium silicate, but it was found that such addition of aluminum alone to molten pig iron in advance of the CaO still had the following problems. Much splashing of molten pig iron takes place upon the addition of aluminum alone, although the reasons for this are not yet clear. Therefore, operational improvement is desired. Moreover, the injection of CaO after the completion of adding aluminum in advance prolongs the desulfurization time which is, for example, 20 to 35 minutes in a 250-ton mixer ladle car) by the time required for the addition of aluminum (for example, 3 to 5 minutes). Consequently, the temperature of molten iron is lowered, and supplementary energy is required in the subsequent process. The prolonged time promotes a damage to the refractory lining in the mixer ladle car.
Belgian Pat. No. 841,834 discloses a process for desulfurizing molten pig iron, wherein it is stated that it is advantageous to simultaneously conduct desulfurization and deoxidation with a strong deoxidizing agent such as aluminum to increase the efficiency of the desulfurizing agent. The desulfurization can also be improved by removal of oxygen via the reaction of aluminum therewith. However, the simultaneous injection of aluminum as the deoxidizing agent and a desulfurizing agent, until the desulfurization reaction is completed, consumes expensive aluminum very wastefully. On the other hand, a continuous injection of a lesser amount of aluminum to avoid such wasteful consumption considerably lowers the desulfurization efficiency.
Trentini et al., "Journal of the Iron and Steel Institute, France (Papers for Special Meeting)", June (1956), pages 124 to 133, in a paper entitled "Desulfurization of Liquid Pig Iron by Blowing with Lime Powder," discloses the simultaneous addition to aluminum and a desulfurizing agent to improve the desulfurization efficiency. Since this process is based on the simultaneous addition, it has the same problem as in said Belgian Pat. No. 831,834, and aluminum and magnesium of metallic elements are treated as equivalents in the art, and thus the art is basically different from the present invention in technical concept of desulfurization. That is, the art neither indicates nor suggests formation of calcium aluminate and the accompanying desulfurization reaction at all.
It has been found by the present inventors that the prior art technique of simultaneous addition of aluminum and CaO has a considerably lower desulfurization efficiency than the prior art technique of adding aluminum in advance of the CaO. That is, when aluminum is added continuously and simultaneously with CaO, i.e. throughout the CaO addition, the shells of calcium silicate are formed on the surface of CaO before the added aluminum reaches a desired concentration in the molten pig iron, and the formation of calcium aluminate cannot be promoted by the addition of the aluminum. Thus, it is necessary to add the entire amount of aluminum necessary for the desulfurization, that is, the amount necessary for establishing a desired aluminum concentration, at the initial stage of reaction. However, as stated above, this necessitates a means for preventing the splashing.
It is therefore, an object of the present invention to provide a process for desulfurizing molten pig iron characterized by:
(1) excellent desulfurization efficiency, as compared with the conventional process for desulfurizing molten pig iron only by CaO. PA1 (2) excellent desulfurization efficiency, as compared with the conventional process of continuous and simultaneous addition of a mixture of CaO and aluminum, and PA1 (3) stable operation without any splashing of molten pig iron when aluminum is added, prolonged life of the refractory lining of a mixing vessel e.g. a mixer ladle car and shortened treating time, as compared with the conventional process of adding aluminum in advance of the CaO.